The present invention relates to two-stage brake valve.
A two-stage brake valve has two stages with different cross sectional areas which become effective in succession upon an actuation of the valve, in that initially the pressure in a larger diameter fill chamber, is applied in order to bring the valve into initial contact, and then the pressure in a smaller diameter pressure chamber is applied to generate the operating pressure. A fill stage pressure relief valve is connected between the fill chamber and a reservoir. This pressure relief valve has a valve element and a pressure relief piston which acts upon the valve element. The valve element is subject on the one hand to the pressure in the reservoir as well as a spring force and, on the other hand, the pressure in the fill chamber. It opens a connection between the fill chamber and the reservoir, when a pre-determined brake pressure in the fill chamber is reached and exceeded. The pressure relief piston is subject at one end to the pressure in the reservoir and can be subject at its other end to the pressure in the pressure chamber. With a free end it is in contact with the valve element, in order to force the valve element into the open direction with increasing pressure difference between the pressure chamber and the fill chamber.
Such a brake valve is described in U.S. Pat. No. 4,455,831 in which the valve element is configured as a ball which interacts with a valve seat in the valve housing. The valve element opens when the fluid pressure within the fill chamber reaches a certain predetermined target value. The pressure relief piston is a stepped piston whose larger piston section is guided in a cylinder bore and is provided with a circular groove that accepts a seal. The thinner piston section engages the ball with its end face. With increasing pressure in the pressure chamber the pressure relief piston is forced with increasing force against the ball in the direction of opening. In this way a constant operation is to be assured even upon a sudden actuation of the brake. By the use of the seal on the pressure relief piston, however, detrimental effects occur due to friction such as hysteresis and spongy brake response.
EP-A-0 534 309 describes a two-stage brake valve, in which a check valve is arranged between the fill chamber and the reservoir, which contains a ball which is forced against a valve seat by the force of a spring that opens upon a pre-set brake, pressure. In order to keep the required actuating force for the brake valve small and to avoid a sudden jump in the operating pressure during the transition between the fill stage and the pressure stage, the closing force of the spring can be controlled as a function of the pressure in the pressure chamber, in that this pressure acts upon a spool valve which, for its part, unloads the spring. This solution also provides for seals in order to avoid leakages which then brings about the disadvantages noted above.
Accordingly, an object of this invention is to provide a valve which permits a direct response of the brake and a reduced hysteresis performance of the brake.
This and other objects are achieved by the present invention, wherein the valve elements as well as the pressure relief piston are configured as cylinder bodies. Each of these cylinder bodies is guided in an associated cylinder bore of the valve body in the form of a sliding fit. An appropriate clearance fit provides a clearance in which the space between the cylindrical piston and the associated bore is so small that any passage of fluid is largely avoided, so that no sealing element need be used, and nevertheless an easy sliding of the cylindrical piston is possible. The bores are preferably finished with a rubbing awl. Furthermore relatively long cylinder bodies are preferably used. Thereby a long channel is formed between the bore and the cylinder body that has a long sealing length that impedes any penetration by the fluid.
Due to the elimination of the sealing elements the valve reacts in its response immediately to actuating forces. The hysteresis effect is reduced considerably and the braking force can be modulated better. Furthermore the valve includes a fill stage pressure relief valve which provides a very simple, cost effectively manufactured configuration consisting of few components.
Preferably, at least one of the two cylinder bodies is a needle bearing roller which is a commercially available high precision component that is a mass-produced item commercially available at low cast. Since needle bearing rollers are usually relatively long, a long sealing length results that, together with the precision fit of the finely finished bore has a sealing effect that can seal adequately without supplementary sealing elements, in particular without elastomeric seals.